This could be an issue that the processor is overheating. If it is they will often kick into power saving mode because it slows down the processor causing it to generate less heat. This is to prevent damage to the computer.

You can check if this may be the problem by using a utility like speedfan to check the temperature of the CPU.

The first thing you should do is have the fan replaced and possibly have the heat sink cleaned and re-greased (with new thermal paste).